How to access array elements in ajax response received from PHP?

jQuery code of ajax function is as follows:

$(document).ready(function() {
$("#zip_code").keyup(function() {
    var el = $(this);
    var module_url = $('#module_url').val();

    if (el.val().length === 5) {
      $.ajax({
        url : module_url,
        cache: false,
        dataType: "json",
        type: "GET",
        data: {
            'request_type':'ajax', 
            'op':'get_city_state',
            'zip_code' : el.val()
        },
        success: function(result, success) { alert(result.join('\n'));
          $("#city").val(result.place_name);
          $("#state_code").val(result.state_code);
        }
      }); 
    }
  });
});

PHP code snippet is as follows :

case "get_city_state":

      // to get the city and state on zip code.
      $ret = $objUserLogin->GetCityState($request); 

      if(!$ret) { 
        $error_msg = $objUserLogin->GetAllErrors();
        list($data) = prepare_response($request);
        $smarty->assign('data', $data);    
      } else {
        $data = $objUserLogin->GetResponse();

        echo $data;
      }     

      die;
      break;

In PHP code the $data contains data in following manner :

<pre>Array
(
    [id] => 23212
    [zip_code] => 28445
    [place_name] => Holly Ridge
    [state_code] => NC
    [created_at] => 1410875971
    [updated_at] => 1410875971
)
</pre>

From the above data(i.e. response which will be available in variable result in ajax response) I want to access only two fields place_name and state_code.

I tried printing the content of result variable using alert(result) in console but I get the word Array

How to achieve this is my doubt?

Thanks in advance.

You should encode your result to json. So instead of the statement echo $data

use

echo json_encode($data);

it will return your result in json format. like

{"id":23212,"place_name":"Holly Ridge"...}

and in your javascript your can access your data